1. Field of Invention
The invention relates to a structure of inkjet-head chip and its manufacturing method. In particular, the invention relates to a structure of inkjet-head chip and its manufacturing method for low inkjet power uses.
2. Related Art
With several breakthroughs in the printing technology of inkjet printers, there are higher demands in better quality and resolution of printing. To achieve higher printing resolutions, the size of ink droplets has to be smaller. Under the same conditions, however, the higher the resolution is the lower the printing speed will be. In order to simultaneously increase the printing speed and resolution, a practical solution is to increase the number of nozzles on a single inkjet-head chip.
To achieve the goal, it is common to integrate driving elements with switches and active characters, such as transistors, with inkjet actuators onto a single inkjet-head chip. The number of packaging contact points X and the number of nozzles Y on the inkjet-head chip is increased from one-to-one (X=Y) to one-to-many (Y=(X/2)2). Such integrated drive head chips (such as those using thermal bubble to drive ink droplets) are usually made by serially connecting metal oxide semiconductor field effect transistor (MOSFET) with an inkjet actuator. The inkjet actuator is the resistor for heating the ink. Such a resistor is called the thermal resistor. The external contact points and the thermal resistor thus render a one-to-many mode. The thermal resistor heats up the ink to produce bubbles, which push ink droplets out. In order for the driving element to provide sufficient power, the resistance of other circuits has to be reduced so that the resistance of the thermal resistor is close to that of the whole loop. Most of the power concentrates on the thermal resistor to be converted to heat. Therefore, it can produce a better bubble generating efficiency.
To focus the power on the thermal resistor, a common method is to utilize field effect transistors (FET) with a larger channel width/length ratio (aspect ratio) to reduce the serial parasitic resistance. Nevertheless, the area occupied by the FET with a large aspect ratio is often much greater than other elements in the chip. In order to increase the resolution, one wants to minimize the FET area. This inevitably adds the system parasitic resistance other than the thermal resistor. A preferred solution is to increase the resistance of the thermal resistor, especially for the inkjet-head with smaller ink droplets because the power needed to eject a singlet droplet is smaller. The power is proportional to the product of the square of voltage and the thermal resistance (Rheater), but inversely proportional to the square of the sum of the thermal resistance and the parasitic resistance (Rheater+Rparasitic)2. Therefore, P=VPP2×Rheater/(Rheater+Rparasitic)2. If the voltage provided by the printer is not increased, increasing the thermal resistance will lower the power generated by the thermal resistor.
One solution for this problem is to reduce the thickness of the interlayer insulator above the thermal resistance, lowering the heat loss from the thermal resistor to the ink. The interlayer insulator is usually made of Si3N4 and SiC. However, one needs to make a second conductive layer and a passivation layer after the interlayer insulator. The interlayer insulator has to be completely insulating in order to separate the circuits in the inkjet-head chip. The insulating property of the interlayer insulator thus affects the yield of the inkjet-head chip. The interlayer insulator above the thermal resistor is where the thermal bubble inkjet-head and the ink have a contact. Therefore, it needs a passivation layer to separate the ink. To overcome the bubble-collapsed force and the chemical properties of the ink over a long time, the passivation layer has to use materials with high melting points, being chemically stable and robust (such as Ta). These passivation layers have to employ high-energy dry etching, active ion etching or wet etching with strong acids or oxidants. Such kinds of etching can easily break the insulation of the passivation layer. If one reduces the thickness of the passivation layer, the damages will be more serious.